Thermionic valve circuits



Patented Oct. 23, 1951 THEBMIONIC VALVE CIRCUITS Roland William Elbourn, Southali, England, as-

lig'uor to Electric & Musical Industries Limited, Hayes. England, a company of Great Britain Application August 5, 1947, Serial No. 766,818 In Great Britain June 19, 1948 Section 1, Public '1. too. Aunm a, 1m

Patent expires June 19, 1988 This invention relates to modulating circuit arrangements of the kind (hereinafter referred to as "the kind described") comprising a pair of thermionic valves having their cathodes connected to a common cathode impedance.

In the circuits of this kind, if a signal is applied to the control electrode of one of said valves while the control electrode of the other valve is maintained at a relatively fixed potential. or if a signal is applied in push-pull to the control'electrodes of the respective valves, a signal corresponding to the difference of potential of the two control electrodes will appear in each anode circuit. The amplitude of this signal will depend, apart from the constant circuit parameters, on the amplitude of the input signal and the mutual conductance of the combination of valves. The mutual conductance will in turn depend mainly upon the mean cathode current of the valves, and by varying the mean cathode current, modulation of the signal in the anode circuits will occur.

The object of the invention is to provide an improved circuit in which modulation is effected in this way.

According to the present invention there is provided a circuit arrangement of the kind described comprising a modulating valve whose anode-to-cathode path is connected in series with the cathodes of the valve pair so that the anode current of said modulating valve serves to modulate the valve pair at their cathodes.

According to another feature of the invention, there is provided a circuit arrangement of the kind described wherein a modulating valve is provided whose anode-to-cathode path is connected in series with the cathodes of said valve pair, and wherein modulating signals independent of signals derived from said valve pair are applied to the. control electrode of said modulating valve so that the anode current of said modulating valve serves to modulate said valve pair at their cathodes.

The modulating valve thus constitutes part at least of said common cathode impedance. A modulating signal applied to the control electrode of the modulating valve will vary the mean cathode current of the valve pair, since this cathode current is the anode current of the modulating valve, thus producing the desired modulation.

The signals in the anode circuits of the valve pair will be liable to contain a component due to the fact that the anode load impedance of the valve pair will constitute part of the anode load of the modulator valve and in some applications of the invention it may be desirable to balance 14 Claims.' (c1. sac-ca) out this component. Therefore, a preferred feature of the invention comprises means for applying to the output impedance of one or each of the valves of said pair, potentials dependent on modulating signals applied to said modulating valve so as to balance out undesired potential variations across said impedance, due to said impedance being included in the anode circuit of said modulating valve.

The invention may, for example, be employed for obtaining an amplitude-modulated output or a frequency-modulated output, or for obtaining an output consisting of width-modulated pulses, and desirably the circuit is so operated that the cathode current of the valve-pair is limited to a small value, so as to produce substantial linearity of modulation. Preferably also, the valves of the pair have screening electrodes at relatively low screen potential, so as to improve still further the linearity of modulation. By operating the valve pair at a low value of cathode current and screen potential the characteristics of the valves of said pair are made nearly exponential, so that the change of mutual conductance which occurs with variation in the cathode current is substantially linear, and for a given change of mutual conductance the change of cathode current required is relatively less. The absolute value of mutual conductance is also greater at low screen potentials.

In order that the said invention may be clearly understood and readily carried into effect, the same will now be more fully described with reference to the accompanying drawings, in which:

Figures 1 to 3 illustrate examples of modulator circuit arrangements according to the invention,

Figure 4 illustrates an amplitude modulated oscillator according to the invention,

Figure 5 illustrates a frequency modulated oscillator according to the invention, and

Figure 6 illustrates a circuit according to the invention for producing width modulated pulses.

Referring to Figure 1, the modulator circuit arrangement comprises two tetrode valves i and 2 having their cathodes connected to a common cathode impedance, comprising in this case a further tetrode 3 which constitutes a modulating valve. The valve 3 has its anode-to-cathode path in series with the cathodes of the pair I, 2, so that the anode current of the valve 3 is arranged to modulate the valves l, 2 at their cathodes. The anode load resistances of valves i and 2 are indicated at l and 5, the output being taken from one or both of these resistances.

It will be assumed that high frequency signals are applied in push-pull to the control electrodes of the valves I and 2, in each case via a coupling cond nser and resistance as shown, and that the output is amplitude-modulated by a low frequency signal input applied to the control electrode of valve 3, also via a coupling condenser and resistance as shown. If the output is taken from both of the resistances 4 and l, a push-pull output is obtained.- The control electrodes of the valves I, 2 and 3 are connected via the resistances shown to points of relatively fixed bias potential, for example earth potential, the anodes of the valves I and 2 being connected in that case to a potential of about 300 volts positive, and the cathode of the valve 3 being connected to a potential of about 300 volts negative with respect to earth potential. If necessary, means may be provided for adjusting the bias potentials applied to the control electrodes oi the valves I and 2 differentially so that the valves can be arranged to operate over parts of their characteristics which are identically shaped. In addition to the modulation produced by the input to valve 3, which varies the mean cathode current to the valves I and 2 and so varies the gain of these valves, the signal in the anode circuits of I and 2 will contain a low frequency component in pushpush, which may be undesired, due to the resistances I and being part of the anode load of valve 3. In addition, in the simple circuit of Figure 1, the non-linear characteristic of valve 3 will interfere with linear modulation.

Figure 2 illustrates a modification of the circuit of Figure 1 designed to eliminate these two undesired features. In this circuit the modulating tetrode 3 is paired with a tetrode 6, and the cathodes of valves 3 and i are connected through a common resistance 30 to the negative potentidl source. In addition the cathode circuits of the valves 3 and 6 include resistances I3 and II which linearize the current waveform of these valves. Reslstances i3 and H may be such that they largeLv determine the curve relating the anode current to the control electrode potential for the valves 3 and 8, and if necessary adjustment may be made to compensate for a slight flattening which occurs in the cathode current mutual conductance curve of valves I and 2 at higher current values.

As before, it will be assumed that high frequency signals are applied to the control electrodes of valve I and- 2 in push-pull, and modulated by a low frequency signal, applied in this case in push-pull to the control electrodes of both of the valves 3 and i, the control electrodes of the valves I, 2 and 3, 6 being connected asin Figure 1 via resistances to points of relatively fixed bias potential. A single output is taken from point ID. The anode load of valve 2 in this case is constituted by resistances 5 and I, and a potentiometer 8 included at the junctions of 5 and I. The anode of the valve 6 is connected in a feedback circuit through a resistance 2! to the tapping on the potentiometer 8 at a point normally half-way down the anode load of valve 2. The arrangement is such that any increase, say, of anode current in valve 3 is accompanied by a corresponding decrease in anode current of valve 8. Neglecting screen currents, half the anode current of valve 3 flows through the anode load 3, I. and 3 of valve 2 and gives rise to the low-frequency component of the modulated si nal. However, this component of the potential waveform at I0 is balanced out by the decrease in anode current of valve 8 applied half-way down the anode load of valve 2. The potentiometer 3 allows adjustment for slight diiferences in the valves 8, 3. and for the screen current of valve 2.

The circuit 01 Figure 3 is similar to that of Figure 2, but the modulator is adapted to deliver the output in push-pull from the anode loads of the valves I and 2 respectively. Therefore, the anode current of valve 8 is applied by two feedback circuits in part to the anode load oi each of valves I and 2, a resistance 21 being included in each anode lead of the valve 8. Since approximately halt the anode current of valve 3 will appear in each anode load l and 5 and cause an undesired modulation in push-push in the outputs taken from said anode loads. the anode of the valve 6 is connected in this case to points at or near the anode end of the loads for valves I and 2, potentiometers 23 and 23 being provided to allow for some adjustment. The valve 3 thus delivers to the anode loads of the valves I and 2 a push-push modulation which is the inverse of said undesired modulation, so that the undesired modulation is balanced out.

In Figure 2, resistance 21 Is included to prevent an excessive D. C. potential on the anode of valve 2, and may be omitted if there is no risk of this.

In the circuits illustrated, the input signals to the valves I and 2 are desirably kept at low level in order to avoid distortion. The circuits according to Figures 2 and 3 are capable of giving very linear modulation. Maximum linearity will be obtained by operating the valves I and 2 at a small cathode current and low screen potential and so taking advantage of the natural exponential characteristics obtained for low currents.

The circuits described in Figures 1 to 3 may be applied to the production of special waveforms by the combination of one waveform applied to the valve pair with another waveform applied to the modulating valve. For example in one application of the circuits, say the circuit of Figure 3, to television scanning circuits, sawtooth waveform potentials of line frequency are applied to the control electrodes of the valves I and 2 in push-pull while sawtooth waveform potentials of frame frequency are applied to the grids of the valves 3 and 6 likewise in push-pull, in order to produce the well known keystone modulation of the line frequency waveform. The circuits may also be applied to the utilisation of square modulation pulses so as to vary the gain of a stage from zero, or substantially zero, to some finite value. Circuits such as described may also be applied to forming the product of waveforms. For example, if two waveforms A and B are applied in push-pull to the control electrodes of the valves I and 2 arranged as in Figure 2, and two further waveforms C and D are applied in pushpull to the control electrodes of the valves 3 and 6, it can be shown that an output can be obtained at the point III which may be expressed by the relationship where a, b and k are constants, the signs of the term depending upon whether the waveforms A and B are applied respectively to the valves I and 2, or vice versa, and whether the waveforms C and D are applied respectively to the valves 3 and 6,or vice versa. If k is made equal to a, the term (C--D) will balance out in the result.

The circuit illustrated in Figure 4 is a modification of the circuit illustrated in Figure 2, a

more tuned circuit comprising an inductance SI and condenser 32 replacing the anode lead of the valve I. The anode of the valve I is also connected to the control electrode of the valve 2 via a condenser and resistance 34, the said resistance being taken to a point of relatively fixed potential. The control electrode of the valve I is also connected to a point of relatively fixed potential. The valve pair I and 2 is thus connected as an oscillator such as described in U. 8. Patent No. 2,300,996, and the presence of the modulator valve 8 with its anode connected to the valves I and 2 enables an amplitude-modulated output to be obtained at the anode of the valve I or at the anode of the valve 2, For example, if a signal of sawtooth waveform is applied tothe control electrode of the valve 3 while the control electrode of valve 6 is maintained at a relatively fixed potential, pulses modulated in amplitude in accordance with the sawtooth waveform can be obtained at Ill and employed, say, for testing purposes. Alternatively or additionally, a sinusoidal oscillation which is amplitude-modulated in accordance with the sawtooth waveform can be obtained at the anode of the'valve I. Oscillations modulated in this way and after being applied to a suitable phase shifting device would be suitable, for example, for producing spiral scanning deflection of the electron beam in a cathode ray tube. of course,- modulating signals of other than sawtooth waveform may be applied to the control electrode of the valve 3.

The valve 6 for balancing out the undesired low-frequency component in the output obtained at the anode of the valve 2, may be omitted where the output is obtained across the tuned circuit in series with the anode of the valve I. In a further modification, a wide band tuned circuit or filter may be connected in the anode circuit of the valve 2, arranged to prevent the undesired low-frequency component from appearing in the output, the valve 6 being again unnecessary.

In the circuit illustrated in Figure 5 the valves I and 2 are arranged to form a multivibrator, the anode of the valve I being coupled to the control electrode of the valve 2 by means of a condenser 35 and a resistance 36 which is taken to a point of relatively high positive potential, which may be the positive terminal of the anode potential source for the valves I and 2. The control electrode of the valve..2 is connected to earth potential, or other suitable relatively low potential, via a diode 31. The anode of the valve 2 is similar y coupled to the control electrode of the valve I by means of condenser 38, resistance 39 and diode 40. The modulator valve 3 in this case enables a frequency modulated output to be obtained from the multivibrator, modulating signals applied to the control electrode of the valve 3 causing the amplitude of the potentials on the control electrodes of the vaves I, 2 to vary in dependence upon the modulating signals. The time taken to charge or discharge between the critical potentials by the time constant circuits of the multivibrators is therefore varied in dependence upon the moduating signals and hence the frequency of the multivibrator is varied. The output of the multivibrator will be amplitudemodulated as well as frequency-modulated, but

this should not be a serious disadvantage. The circuit illustrated may be employed for frequency-modulating carrier oscillations of, for example, 5 megacycles per second, the frequency being kept stable by periodic reference to a master oscillator. If desired, a wide band tuned circuit or filter may be connected in the anode circuit of that one of the valves I and 2 from which the output is obtained, for the purpose described with reference to Figure 4.

In the circuit illustrated in Figure 6 the valves I and 2 are arranged to form a "flip-flop," that is to say a relaxation circuit having one stable condition and one unstable condition, the anode of the valve I being connected to the control electrode of the valve 2 via a condenser II, resistance 42 and diode 43. The circuit is arranged to generate pulses which can be obtained at the anode of the valve 2 and which can be widthmodulated in dependence upon modulating 818- nals applied to the control electrode of the modulating valve 3. The circuit may be employed. .for example, when it is desiredto transmit information by modulating the width of synchronising pulses in television broadcasts, controlling pulses of the frequency of the synchronising pulses being applied in a positive sense to the control electrode of the valve I via a condenser and the resistance 45 which is shunted by the diode 46 as shown. The leading edge of a controlling pulse will be transmitted to the control electrode of the valve I as a positive pulse which will trigger the flip-flop," renderin the valve I conducting and'the valve 2 non-conducting. The output of the circuit is taken from the anode load 5, I, 8 of the valve 2 and the duration of the resultant positive pulse at the anode of the valve 2 will be determined by the amplitude of the potential applied to the control electrode of the valve 2, which in turn is determined by the modulating signals applied to the control electrode of the modulating valve 3. The diode It prevents the trailing edge of a controlling pulse from appearing on the control electrode of the valve I as a negative pulse of sufllcient amplitude to restore the flip-flop premature y to its stable condition, when the width of the output pulse is greater than the width of the controlling pulses. The valve 6 may be omitted in this application of the invention, since it may be preferable to remove the undesired low frequency modulation of the output of the circuit, which will be of the nature of a direct current variation, by means of a resistance-capacity coupling, the output pulses being thereafter levelled to a desired direct current level by known direct-current reinsertion practice.

What I claim is:

1. A circuit arrangement, comprising a pair of pair of output eectrodes and each having an output circuit including its output electrodes, a first electron discharge device having at least an anode and a cathode and having a cathode-anode space discharge path common to the output circuits of said pair of control devices, a second electron discharge device having at least an anode and a cathode and having a cathode circuit comprising an impedance common to the cathode-anode circuit of said first discharge device, and a feedback circuit between the anode of said second discharge device and the output circuit of at least one of said pair of control devices.

2. A circuit arrangement as defined in claim 1 wherein each of said first and second discharge devices has a control electrode and wherein connections are provided for applying modulating signals antiphasally to such control electrodes.

3. A circuit arrangement as defined in claim 1 to the control electrodes of said pair of controldevices.

- 5. A circuit arrangement as defined in claim 1 wherein the anode of said second discharge device is provided with two leads and wherein said two leads provide two feedback circuits, one between the anode of said second discharge device and the output circuits of each of said pair of control devices.

8. A circuit arrangement as defined in claim 4 wherein the anode of said second discharge device-is provided with two leads and wherein said two leads provide two feedback circuits, one between the anode of said second discharge device and the output circuits of each of said pair of control devices.

'7. A circuit arrangement, comprising a pair of electron discharge devices each having an anode, a cathode and a control electrode and each having an anode-cathode circuit, a third electron discharge device having at least an anode and a cathode and having a cathode-anode space discharge path common to the anode-cathode circuits of said pair of devices, a tuned circuit connected in series in the anode-cathode circuit of one of said pair of devices, and means connecting the anode of said one of said pair of devices to the control electrode of the other of said pair of devices, whereby the two devices of said pair are connected together as an oscillation generator.

8. A circuit arrangement, comprising a pair of electron discharge devices each having an anode, a cathode and a control electrode and each having a cathode-anode circuit, a third electron discharge device having at least an anode and a cathode and having a cathodeanode space discharge path common to the anode-cathode circuits of said pair of devices, a fourth electron discharge device having at least an anode and a cathode and having acathode circuit comprising an impedance common to the cathode-anode circuit of said third device, a feedback circuit between the anode of said fourth device and the output circuit of one of said pair of devices, a tuned circuit connected in series in the anode-cathode circuit of the other of said pair of devices, and means connecting the anode of said other of said pair of devices to the control electrode of said one of said pair of devices, whereby the two devices of said pair of devices are connected together as an oscillation generator. 9. A circuit arrangement as defined in claim 8 wherein each of said third and fourth devices has a control electrode and wherein connections are provided for applying modulating signals antiphasally to the control electrodes of said third and fourth devices.

10. A circuit arrangement, comprising a pair of electron control devices each having a control electrode and a cathode, a common cathode impedance connected to the cathodes of both said devices, control electrode-to-cathode circuits for each of said devices, each of said circuits including as a part thereof said impedance, said impedance comprising the anode-cathode path of a modulating electron discharge device conand a resistor in series with the cathode of said modulating device, said resistor having a value such as to effectively determine the anode current-control electrode potential characteristic of the modulating device.

11. A circuit arrangement, comprising a pair of electron control devices each having a control electrode and a cathode and each being provided with an output circuit, a common cathode impedance connected to the cathodes of both said devices, control electrode-to-cathode circuits for each of said devices, each of said circuits including as a part thereof said impedance, said impedance comprising the anode-cathode path of an electron discharge device connected in series with the pair of control devices, another electron discharge device having at least an anode and a cathode and having a cathode circuit comprising an impedance common to the anode-cathode external circuit of the first-named discharge device, a feed :back circuit between the anode of said other discharge device and the output circult of at least one of said pair of control devices,

- and a separate resistor connected in series with the cathode of each of said discharge devices, each resistor having a value such as to effectively determine the anode current-control electrode potential characteristic of the respective discharge device to which it is connected.

12. A circuit arrangement as defined-in claim 11, wherein means are provided for applying carrier energy anti-phasally to the control electrodes of the two control devices, wherein each of the two discharge devices has a control electrode and wherein means are provided for applying modulating signals anti-phasally to the lastnamed control electrodes.

13. A circuit arrangement, comprising a pair of electron control devices each having a control electrode and a cathode; a common cathode impedance connected to the cathodes of both said devices, control electrode-to-cathode circuits for each of said devices, each of said circuits including as a part thereof said impedance, means for difierently biasing the control electrodes of said pair of devices, said impedance comprising the anode-cathode path of a modulating electron discharge device connected in series with the pair of control devices, said discharge device having a control electrode, means for applying modulating signals to said last-named control electrode to thereby modulate the current in said pair of control devices, a resistor in series with the oathode of said modulating device, a source of repetitive waves connected to the control electrode of one device of said pair of devices, and an output circuit coupled to said one device.

14. A circuit arrangement, comprising a pair of multielectrode vacuum tubes each having at least an anode, a screen grid, a control grid and a cathode, means for biasing the screen grids of said tubes positively with respect to their cathodes, a common cathode impedance connected to the cathodes of both said tubes, control grid-tocathode circuits for each of said tubes, each of said circuits including as a part thereof said impedance, means for differently biasing the control grids of said pair of tubes, said impedance comprising the anode-cathode path of a modulating electron discharge device connected in series with the pair of tubes, said discharge device having a control electrode, means for applying modulating signals to said control electrode to thereby modulate the current in said pair of tubes, a resistor in series with the cathode of said modulating device, a source of repetitive waves connected to the control grid of one tube of said pair of tubes, and an output circuit connected to the anode of said one tube oi said pair of tubes.

ROLAND WILLIAM ELBOURN.

REFERENCES CITED The following references are of record in the flle of this patent:

UNITED STATES PATENTS Number o Number Date Name Urtel Aug. 29, 1939 FOREIGN lATENTS Country Date Australia June 20, 1933 Great Britain Dec. 20, 1933 

